a. Field of Invention
This invention pertains to an improved cover for a suction box used in papermaking machines, wherein water is removed from a moving fabric or web by suction.
b. Description of the Prior Art
Various types of paper may be made at high speed and economically on so-called papermaking machines. In a papermaking machine, a slurry of wood pulp or other materials is dispensed onto an endless forming fabric. The forming fabric with the slurry moves past several suction boxes used to remove water from the slurry to form a continuous paper sheet. Since this sheet still has a high water content, it is usually transferred to a press section where it is contacted with at least one continuous press fabric and fed through press nips for the further removal of water by mechanical compression. The press fabric is then passed over a suction box where water is removed from the fabric. Thus the press fabric may be used in a continuous process.
One common type of suction box is provided with a cover having a slot extending across the full width of the press fabric. The slot is generally in the range of 3/8 to 3 inches in machine direction width. (The suction box is described herein as part of the press section of the papermaking machine, however it should be understood that it may also be used in the paper forming section). This type of suction box has been found to be unsatisfactory for a number of reasons. For example, as the seam of a machine seamable press fabric as described in U.S. Pat. No. 4,601,785 passes over the slot, the fabric makes a loud, unpleasant popping sound which increases the overall noise level produced by the machine. Also, as the seam passes over the slot in the suction box, it rubs against the slot edges causing the seam flap, formed of batt material, to wear out before the rest of the fabric. Thus, the useful life of the press fabric is reduced drastically. The fabric may also fail due to the flexing of the fabric adjacent to the seam due to the movement of the seam caused by vacuuming down into the slot.